Limit Test for Sulphate MCQs With Answer is a high‑yield topic for B. Pharm students preparing for Pharmaceutical Analysis and Quality Control. The pharmacopeial limit test for sulphate (IP/USP/BP) relies on the formation of barium sulfate turbidity when sulphate ions react with barium chloride under controlled acidity, enabling comparison with a standard. Understanding the principle, reagents, procedure, interferences, turbidity comparison, validation, and interpretation is crucial for ensuring excipients and APIs comply with sulphate impurity limits. This resource covers the chemistry (Ba2+ + SO4−− → BaSO4↓), role of acid and alcohol, use of Nessler cylinders, standard sulphate solution, and method sensitivity and troubleshooting—optimized with keywords for exam and industry relevance. Now let’s test your knowledge with 50 MCQs on this topic.
Q1. What is the basic principle of the pharmacopeial limit test for sulphate?
- Formation of a colored complex with sulphate and ferric ion
- Formation of insoluble barium sulfate causing turbidity
- Oxidation of sulphate to sulfate radical detected by UV
- Ion-exchange separation followed by titration
Correct Answer: Formation of insoluble barium sulfate causing turbidity
Q2. Which reagent is essential for precipitating sulphate in the limit test?
- Silver nitrate solution
- Barium chloride solution
- Lead acetate solution
- Mercuric chloride solution
Correct Answer: Barium chloride solution
Q3. Why is the test solution acidified before adding barium chloride in the sulphate limit test?
- To increase the solubility of barium sulfate
- To prevent interference from carbonate and phosphate
- To oxidize sulphate to sulfite
- To reduce chloride interference
Correct Answer: To prevent interference from carbonate and phosphate
Q4. Which acid is commonly used to acidify the solution in the sulphate limit test according to pharmacopeias?
- Dilute hydrochloric acid
- Concentrated nitric acid only
- Perchloric acid
- Boric acid
Correct Answer: Dilute hydrochloric acid
Q5. What visual endpoint is compared in the sulphate limit test?
- Intensity of blue color against a blank
- Height of foam formed
- Degree of turbidity against a standard
- Change in pH after reaction
Correct Answer: Degree of turbidity against a standard
Q6. Which compound acts as the precipitate in the sulphate limit test?
- BaSO4
- Ag2SO4
- PbSO4
- CaSO4
Correct Answer: BaSO4
Q7. What is the role of alcohol (e.g., ethanol) in some sulphate limit test procedures?
- To dissolve the BaSO4 precipitate
- To increase the turbidity sensitivity and stability
- To neutralize excess acid
- To mask chloride interference
Correct Answer: To increase the turbidity sensitivity and stability
Q8. Which glassware is typically used for visual comparison in the sulphate limit test?
- Burettes
- Nessler cylinders
- Conical flasks
- Separating funnels
Correct Answer: Nessler cylinders
Q9. A standard sulphate solution for comparison is commonly prepared using which primary standard?
- Potassium sulfate
- Sodium sulfate decahydrate
- Calcium sulfate hemihydrate
- Magnesium sulfate heptahydrate
Correct Answer: Potassium sulfate
Q10. What is the main purpose of adding acid before BaCl2 in the sulphate limit test?
- To speed up BaSO4 crystal growth beyond detection
- To maintain pH that prevents precipitation of other barium salts
- To bleach colored impurities
- To oxidize organic matter
Correct Answer: To maintain pH that prevents precipitation of other barium salts
Q11. Which analytical technique can be used instead of visual comparison to improve sensitivity in the sulphate limit test?
- Titrimetry with EDTA
- Fluorimetry
- Nephelometry or turbidimetry
- Potentiometry with pH electrode
Correct Answer: Nephelometry or turbidimetry
Q12. What is the effect of EDTA contamination on the sulphate limit test?
- Enhances turbidity by seeding
- No effect on precipitation
- Complexes Ba2+ and decreases turbidity
- Oxidizes sulphate to sulfate radical
Correct Answer: Complexes Ba2+ and decreases turbidity
Q13. Which of the following ions can cause false positive turbidity with BaCl2 if not acidified?
- Nitrate
- Phosphate
- Sodium
- Ammonium
Correct Answer: Phosphate
Q14. Why is a blank (reagent control) prepared alongside test and standard in the sulphate limit test?
- To supply extra Ba2+
- To correct for reagent and water turbidity
- To accelerate precipitation
- To increase ionic strength
Correct Answer: To correct for reagent and water turbidity
Q15. In IP/USP, the turbidity is compared after a specific time interval. What is the purpose of this timing?
- Allows BaSO4 turbidity to develop reproducibly
- Allows complete dissolution of BaSO4
- Ensures pH rises to neutral
- Prevents chloride interference
Correct Answer: Allows BaSO4 turbidity to develop reproducibly
Q16. Which water quality should be used in the sulphate limit test?
- Tap water
- Purified water verified to be low in sulphate
- Any deionized water without verification
- Distilled water from copper condensers
Correct Answer: Purified water verified to be low in sulphate
Q17. What is the significance of comparing the turbidity of the test solution with that of a standard solution containing a known amount of sulphate?
- Quantifies exact sulphate concentration
- Determines limit compliance relative to specification
- Detects presence of chloride
- Measures pKa of sulphuric acid
Correct Answer: Determines limit compliance relative to specification
Q18. In the sulphate limit test, what is the consequence of shaking too vigorously after adding BaCl2?
- Inhibits nucleation entirely
- Causes loss of BaSO4 by adhesion to walls
- Produces inconsistent particle size and variable turbidity
- Immediately clarifies the solution
Correct Answer: Produces inconsistent particle size and variable turbidity
Q19. Which statement best describes the role of ionic strength in BaSO4 precipitation during the test?
- Higher ionic strength promotes controlled nucleation and turbidity
- Ionic strength has no effect
- Higher ionic strength dissolves BaSO4
- Lower ionic strength always improves sensitivity
Correct Answer: Higher ionic strength promotes controlled nucleation and turbidity
Q20. The limit test for sulphate is categorized as which type of test in pharmacopeias?
- Quantitative assay
- Semi-quantitative limit test
- Qualitative identification test
- Stoichiometric titration
Correct Answer: Semi-quantitative limit test
Q21. What is the typical visual reference used to judge compliance in the sulphate limit test?
- Test solution must be more turbid than standard
- Test solution must be less turbid than or equal to standard
- Test solution must be perfectly clear
- Standard must match the blank exactly
Correct Answer: Test solution must be less turbid than or equal to standard
Q22. Which sample preparation step is most important for colored or cloudy samples?
- Direct addition of BaCl2 without adjustment
- Decolorization or filtration prior to the test
- Boiling with strong alkali
- Addition of silver nitrate first
Correct Answer: Decolorization or filtration prior to the test
Q23. Why should sulphate-containing detergents be avoided when cleaning glassware for this test?
- They reduce pH too much
- They can leave residual sulphate causing false positives
- They corrode barium chloride
- They form explosive peroxides
Correct Answer: They can leave residual sulphate causing false positives
Q24. Which of the following can be used to prepare the sulphate standard solution besides potassium sulfate?
- Sulfuric acid of known normality
- Sodium chloride standard
- Potassium iodide primary standard
- Glucose solution
Correct Answer: Sulfuric acid of known normality
Q25. In visual comparison, why are identical cylinders and volumes used?
- To allow more vigorous shaking
- To ensure comparable optical path length
- To reduce reaction time
- To increase temperature uniformity only
Correct Answer: To ensure comparable optical path length
Q26. Which parameter most critically affects BaSO4 particle size distribution during the test?
- Light intensity in the lab
- Order of reagent addition and mixing rate
- Color of the cylinder
- Use of plastic stir bar
Correct Answer: Order of reagent addition and mixing rate
Q27. What is the function of maintaining controlled acidity (often mildly acidic) in the sulphate test?
- Prevents BaSO4 dissolution via complexation
- Minimizes precipitation of barium carbonate and barium phosphate
- Ensures oxidation of sulfite
- Promotes chloride precipitation
Correct Answer: Minimizes precipitation of barium carbonate and barium phosphate
Q28. Which interfering anion is best removed by prior acidification and, if necessary, precipitation before the sulphate test?
- Nitrate
- Carbonate
- Fluoride
- Perchlorate
Correct Answer: Carbonate
Q29. What is the typical observation time window after adding BaCl2 before turbidity comparison?
- Immediately, with no delay
- After a controlled short interval (e.g., about 5 minutes)
- After 1 hour only
- After overnight standing
Correct Answer: After a controlled short interval (e.g., about 5 minutes)
Q30. For improved reproducibility, what temperature condition is preferred during the sulphate limit test?
- Strongly heated solution (near boiling)
- Ice-cold solution
- Room temperature, consistent across test and standard
- Variable temperature to accelerate reaction
Correct Answer: Room temperature, consistent across test and standard
Q31. In a nephelometric variant, the measured signal is proportional to which property of the BaSO4 suspension?
- Absorbance at 280 nm
- Scattered light intensity at a fixed angle
- Refractive index of solvent
- Conductivity of the mixture
Correct Answer: Scattered light intensity at a fixed angle
Q32. Which statement about the standard sulphate solution is correct?
- It must be freshly prepared before every test without storage
- It can be prepared from a primary standard and standardized if stored appropriately
- It can be any solution with turbidity
- It must be prepared in ethanol only
Correct Answer: It can be prepared from a primary standard and standardized if stored appropriately
Q33. If the test solution is more turbid than the standard, what is the conclusion?
- Sample complies with sulphate limit
- Sample fails the sulphate limit test
- Test is invalid and must be ignored
- There is no sulphate present
Correct Answer: Sample fails the sulphate limit test
Q34. Which of the following best minimizes operator bias in visual turbidity comparison?
- Use colored background
- Use consistent lighting and white background
- Use different cylinder diameters
- Shake the test cylinder only
Correct Answer: Use consistent lighting and white background
Q35. Why is glycerol or alcohol sometimes included in the reagent system for the sulphate test?
- To chelate barium ions
- To increase viscosity and stabilize turbidity
- To neutralize acid completely
- To dissolve sulphate ions
Correct Answer: To increase viscosity and stabilize turbidity
Q36. Which sample type typically has the tightest sulphate limit and demands careful control of reagent purity?
- Topical ointments
- Parenteral (injectable) solutions
- Oral capsules
- Dermal patches
Correct Answer: Parenteral (injectable) solutions
Q37. What is the impact of high chloride levels on the sulphate limit test?
- Direct precipitation with Ba2+ to form BaCl2
- No direct precipitation; minimal interference under acidic conditions
- Forms insoluble BaCl precipitate
- Oxidizes sulphate to sulfate radical
Correct Answer: No direct precipitation; minimal interference under acidic conditions
Q38. Prior to comparison, why should both test and standard be treated identically (same reagents, volumes, timing)?
- To maximize turbidity in the test only
- To ensure matrix equivalency and valid comparison
- To reduce the amount of standard required
- To avoid the need for a blank
Correct Answer: To ensure matrix equivalency and valid comparison
Q39. If the sample contains sulfite (SO3 2−), what precaution is necessary before running the sulphate limit test?
- Reduce sulfite further with zinc
- Oxidize sulfite to sulphate to avoid underestimation
- Add EDTA in excess
- Neutralize with ammonia
Correct Answer: Oxidize sulfite to sulphate to avoid underestimation
Q40. What is the appropriate action if visible particulates are present before reagent addition?
- Proceed; particulates do not affect turbidity
- Filter or clarify the solution to remove particulates
- Add more BaCl2 to overcome particulates
- Add strong base to dissolve solids
Correct Answer: Filter or clarify the solution to remove particulates
Q41. Which best describes the chemical reaction in the test?
- Ba2+ + SO4 2− → BaSO4(s)
- Ba2+ + Cl− → BaCl(s)
- Ba2+ + NO3− → Ba(NO3)2(s)
- Ba2+ + PO4 3− → Ba3(PO4)2(aq)
Correct Answer: Ba2+ + SO4 2− → BaSO4(s)
Q42. Which statement about turbidity measurement is true?
- Nephelometry measures transmitted light directly
- Turbidimetry measures decrease in transmitted light due to scattering
- Nephelometry is unaffected by particle size
- Turbidimetry is independent of concentration
Correct Answer: Turbidimetry measures decrease in transmitted light due to scattering
Q43. What is a common suitability criterion for the sulphate limit test when using instrumental detection?
- Linearity of slope vs. temperature
- Standard-turbidity response meets acceptance within defined range
- Color development must be violet
- pH must be exactly 7.0
Correct Answer: Standard-turbidity response meets acceptance within defined range
Q44. What is the main reason for using matched Nessler cylinders?
- They are cheaper than beakers
- They ensure identical optical characteristics for comparison
- They prevent CO2 absorption
- They eliminate need for timing
Correct Answer: They ensure identical optical characteristics for comparison
Q45. If excessive barium chloride is added, what is the likely effect?
- No effect; any excess is harmless
- Peptization and non-uniform turbidity formation
- Complete dissolution of BaSO4
- Oxidation of organic impurities
Correct Answer: Peptization and non-uniform turbidity formation
Q46. Which is the best practice to minimize variability between runs?
- Use different reagent lots for each run
- Control reagent volumes, mixing, time, and temperature strictly
- Skip the blank
- Use tap water for convenience
Correct Answer: Control reagent volumes, mixing, time, and temperature strictly
Q47. In the context of pharmacopeial compliance, the sulphate limit is typically expressed as:
- Exact molarity of sulphate in solution
- Maximum allowed turbidity units only
- Percent or ppm of sulphate impurity relative to sample
- Milliequivalents of barium used
Correct Answer: Percent or ppm of sulphate impurity relative to sample
Q48. What is the purpose of using a standard solution containing a known microgram amount of sulphate in the comparison cylinder?
- To act as a catalyst for precipitation
- To define the visual reference corresponding to the specification limit
- To neutralize acid in the test
- To remove chloride interference
Correct Answer: To define the visual reference corresponding to the specification limit
Q49. Which step helps prevent false turbidity due to environmental dust or fibers?
- Using unfiltered air flow across cylinders
- Covering cylinders or using stoppers during development
- Placing cylinders near fans
- Performing the test outdoors
Correct Answer: Covering cylinders or using stoppers during development
Q50. If both the test and standard appear more turbid than the blank immediately upon reagent addition, what is the best next step?
- Proceed with comparison; this is normal
- Investigate reagent contamination and remake blank, reagents, and standards
- Add more barium chloride until clear
- Ignore the blank and decide visually
Correct Answer: Investigate reagent contamination and remake blank, reagents, and standards
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